XORG CONFIGURATION

See the Beginners Guide for details

using the ATI catalyst driver

pacman -S catalyst

using the open source driver

You have the choice between the radeon and radeonhd driver.
However you need a new DRM Kernel Module to get direct rendering.
The new DRM module should be included in Kernel 2.6.26 so either updgrade
or get the libdrm-git package from the AUR.
You should also install XOrg Server from git using the xorg-server-git
package from the AUR. (this will take some time).
Dont forget xf86-input-mouse-git and xf86-input-keyboard-git.